In U.S. Pat. No. 3,653,732, assigned to the same assignee as the present application, there is disclosed a multi-ball pivot assembly for use in precision bearing applications such as gyroscope suspension systems. Since only very small angular rotations are encountered in such systems, there is little concern for control of running friction and justifiably great concern for control of starting friction and random torque effects. Such friction errors have been traced to such factors as raceway misalignment and imperfections. The assembly of the cited patent utilizes no inner raceway and the balls are in direct contact with the surface of a rotatable shaft. As well, no conventional outer raceway is used. A right cylinder serves as the outer ring against which the balls bear. The balls are constrained axially by a pair of flat washers attached to the outer ring. Eight balls substantially fill the annular volume defined by the shaft, the outer ring, and the washers. The elimination of raceways and conventional ball retainers avoids the friction problems usually associated with these elements. Only point contact between structural elements remains. Suitably hard materials tooled to instrument quality surface finish further reduce friction. Nominal radial clearances are allowed but tolerances on diameters, roundness, and finish are controlled so that total radial shake under all orientations does not result in either an interference (zero radial play) or excessive looseness.
Friction control in the bearing assembly of the cited patent contributes to gyroscope precision superior to any previously known bearing assemblies. However, that assembly like others then available, was developed without recognition of the significance of the density of the balls as related to the density of the fluid lubricant that fills the assembly.
Some balls of less density than those of conventional steel bearings have been used; for example, hollow or glass balls. However, their use was for other reasons. That there has been no serious consideration of matching densities may be attributable to a lack of awareness that a match reduces frictional uncertainty in the bearing assembly. Tungsten carbide balls, with a specific gravity of 14.8 grams per cubic centimeter were used in the assembly of the cited patent. They met known requirements for the ball bearings, including smoothness and hardness. Additionally, the extreme small size of the balls, preferably 0.0078 inches in radius, restricted the choice of acceptable materials to those which could be machined to the required specifications. The fluid lubricant of the assembly, usually bromotrifluoroethylene, has a specific gravity of 2.3 grams per cubic centimeter and was likewise chosen for its known lubricating and other qualities. The difference of density between balls and fluid produced an effective floated weight for each ball of 0.399 dynes. Concentration on the multiplicity of requirements that ball and lubricant materials respectively had to meet probably blinded developers from correlating a density match and a friction reduction. Within the assembly, the uncertainty force of sliding friction was present and tolerated for lack of knowledge of a method of reducing it.